Consumer electronics devices are continually getting smaller and, with advances in technology, are gaining ever-increasing performance and functionality. This is clearly evident in the technology used in consumer electronic products and especially, but not exclusively, portable products such as mobile phones, audio players, video players, personal digital assistants (PDAs), various wearable devices, mobile computing platforms such as laptop computers or tablets and/or games devices. Requirements of the mobile phone industry for example, are driving the components to become smaller with higher functionality and reduced cost. It is therefore desirable to integrate functions of electronic circuits together and combine them with transducer devices such as microphones and speakers.
Micro-electromechanical-system (MEMS) transducers, such as MEMS microphones are finding application in many of these devices. There is therefore also a continual drive to reduce the size and cost of the MEMS devices.
Microphone devices formed using MEMS fabrication processes typically comprise one or more membranes with electrodes for read-out/drive that are deposited on or within the membranes and/or a substrate or back-plate. In the case of MEMS pressure sensors and microphones, the electrical output signal read-out is usually accomplished by measuring a signal related to the capacitance between the electrodes.
To provide protection the MEMS transducer will be contained within a package. The package effectively encloses the MEMS transducer and can provide environmental protection and may also provide shielding for electromagnetic interference (EMI) or the like. The package also provides at least one external connection for outputting the electrical signal to downstream circuitry. For microphones and the like the package will typically have a sound port to allow transmission of sound waves to/from the transducer within the package and the transducer may be configured so that the flexible membrane is located between first and second volumes, i.e. spaces/cavities that may be filled with air (or some other gas suitable for transmission of acoustic waves), and which are sized sufficiently so that the transducer provides the desired acoustic response. The sound port acoustically couples to a first volume on one side of the transducer membrane, which may sometimes be referred to as a front volume. The second volume, sometimes referred to as a back volume, on the other side of the one of more membranes is generally required to allow the membrane to move freely in response to incident sound or pressure waves, and this back volume may be substantially sealed (although it will be appreciated by one skilled in the art that for MEMS microphones and the like the first and second volumes may be connected by one or more flow paths, such as small holes in the membrane, that are configured so as present a relatively high acoustic impedance at the desired acoustic frequencies but which allow for low-frequency pressure equalisation between the two volumes to account for pressure differentials due to temperature changes or the like).
Various package designs are known. For example, FIGS. 1a and 1b illustrate “lid-type” packages 100. A MEMS transducer 101 is mounted to an upper surface of a package substrate 102. The package substrate 102 may be PCB (printed circuit board) or any other suitable material. A cover or “lid” 103 is located over the transducer 101 and is attached to the upper surface of the package substrate 102. The cover 103 may be a metallic lid. In FIG. 1a, an aperture 104 in the cover 103 provides a sound port and allows acoustic signals to enter the package. In FIG. 1b an aperture 104 in the substrate 102 provides the sound port and the MEMS transducer is mounted such that the flexible membrane of the transducer extends over the sound port.
The package may also contain an integrated circuit (not shown). The integrated circuit will typically be formed on a die of semiconductor material and will be customised for a particular application. The integrated circuit will be electrically connected to electrodes of the transducer 101 and an electrically conductive path will be provided between the integrated circuit and an electrical connection provided on an external surface of the package. The integrated circuit may provide bias to the transducer and may buffer or amplify a signal from the transducer.
FIG. 2 illustrates an alternative package type known as a “laminate to laminate” package. The package shown in FIG. 2 comprises a first member 201 comprising a FR-4 board core having a solder mask stop layer applied to the upper and lower surfaces thereof, a second member 202 disposed in a plane overlying the first member and comprising an FR-4 board coated on an inner/lower surface thereof with a solder stop layer 111, and a third member 203 (or “interposer member”) which is interposed between the first and second members. The third member forms at least a part of the side walls of the package. The third member can be considered to comprise a cavity or void such that, when the three members are bonded together e.g. by means of solder bonds 110, a space or chamber 204 is formed between the lower surface of the second member 202 and an upper surface of the first member 201, wherein the side walls of the chamber are partially provided by the cavity edges of the third member 203. A transducer 101 and an integrated circuit 106 are provided within the chamber 204.
Although several different arrangements are known, according to the FIG. 2 arrangement a port hole 104 extends through the first member of the package and an external electrical connection 105, which may for example comprise solder pads or the like, is provided on the outer surface of the second member. According to convention, the configuration shown in FIG. 2—in which the sound port is provided on opposite side of the package to the external electrical connection—is known as a “top port” configuration. It will be appreciated that the term “top port” does not imply any particular orientation of the package device either during manufacture, processing or any subsequent application. In this example, the acoustic port 104 is provided by means of a cavity comprised in the first member which, according to the orientation of the package shown in FIG. 2, is actually illustrated as being beneath the second member.
In the top port arrangement of FIG. 2, the transducer 101 is supported in a fixed relationship with respect to the first member 201 and is arranged such that the flexible membrane of the transducer extends over—or overlies—the acoustic port 104. The transducer is connected to an integrated circuit 106 which is also supported by the first member. Specifically, the transducer and the integrated circuit are mounted on the solder stop mask layer 111 provided on the upper surface of the first member. It will be appreciated that in order for an electrical signal generated by the integrated circuit to be output from the package, an electrically conductive path must be provided from the integrated circuit 106, which is mounted on an inner surface of the first member, to the external electrical connection 105, which is provided on an outer surface of the second member 202.
As shown in FIG. 2, this is achieved by means of a plurality of conductive vias (vertical interconnected access) 109a and 109b which are provided through the third member 203 and the second member 202 respectively. The vias allow an electrical connection to be made from the lower plane of the integrated circuit up to the upper plane of the external electrical connection 105. Furthermore, an electrical contact 112 on the upper surface of the first member and a conductive path across the first member (in this illustrated example including an embedded horizontal portion) to a metal solder pad 117 are provided to form an electrically conductive path from the integrated circuit 106 to a solder bond 110v which is directly connected from pad 117 to the bottom of the via 109a. 
From consideration of FIG. 2, and also from FIG. 3 which shows a view from above of a cross-section taken through the plane of the third member, it will be appreciated that the lateral dimensions of the inner chamber of the package that is formed between the edges of the cavity of the third member will depend on the width S of the sidewalls formed by the third member. Moreover, it can be seen that the portion of the third member which incorporates the vias exhibits a greater width W than the other side walls of the package. Thus the lateral dimensions, i.e. the width of the portion of the chamber that is defined between the side walls formed by the third member, is limited for a given external package size by the need for a minimum side wall width to provide a required mechanical strength and the provision of side wall vias which result in a wider side wall dimension along one or more of the side walls. This is further compounded by the provision of the electrical contact 112 and pad 117 and the need for an intervening solder dam of material 111 which all occupy lateral space on the first member.
It will be appreciated that the restrictions on chamber size which arise a consequence of the manner in which an inter-planar electrical connection is provided from the lower plane of the first member to the upper plane of the second member, also restricts the size of the integrated circuit die which can potentially limit the functionality of the integrated circuit.
Although it would be possible to compensate for the lateral space occupied by the wider side-wall by increasing the overall footprint of the package, this would increase the materials and production cost of each package. The larger package might render the product unsuitable for some end-user applications, for example earbuds, where space constraints are particularly severe. Also it will be appreciated that many industrial processes for manufacturing MEMS transducer packages utilise a standardised package configuration wherein the external dimensions of the package, including the footprint, are fixed. This approach offers a number of distinct advantages in terms of economies of scale and automation during manufacturing and/or assembly.